The present invention relates generally to magnetic sensing devices, and more particularly to magnetic sensing devices having means for compensating for offset in the output signal of a magnetic sensor.
Magnetic rotational position sensors have been extensively incorporated in engine timing systems of motor vehicles to ascertain the rotational position of a rotary shaft relative to a referenced position of the shaft. A magnetoresistive (MR) sensor having two MR elements in a half-bridge or differential configuration, i.e., with the elements connected in series and supplied with a constant DC voltage, generates an output signal at the junction between the two elements that varies in amplitude as a function of the relative resistance values of the MR elements, and thus varies as a function of variations in magnetic field strength according to the well known magnetoresistive effect. A half-bridge MR sensor positioned adjacent to the teeth of a gear in an electronic ignition timing system, for example, may generate an analog output signal that varies sinusoidally as the teeth pass the sensor in the presence of a magnetic filed. The average or DC value of the sine wave is ideally equal to half the supply voltage. That is, the two MR elements in the half-bridge are ideally equal in resistance when they are equidistant from the center of a tooth or a gap between teeth on the target wheel, such that the output voltage at each such point is half the supply voltage. In this ideal state the output voltage can be compared to a fixed reference voltage equal to half the supply voltage to obtain a digital signal having a 50% duty cycle.
Offsets can occur, however, if the two MR elements are not perfectly matched and aligned or if they experience changes in temperature or other ambient conditions. Temperature compensation is provided to some extent by the bridge configuration itself, but offsets can still occur because, for example, the temperature coefficients of the elements are not necessarily equal. Capacitive (or AC) coupling and other techniques have been employed or proposed for offset compensation in certain applications, but the known techniques are not without disadvantages, such as the inability of an AC-coupled sensing device to reliably sense motion at low speeds. A need therefore remains for improvements in means for compensating for offset in the output signals of magnetic sensors.
The present invention overcomes a number of disadvantages of the prior art and provides improved means for compensating for offset in the output signals of magnetic sensors.
According to one aspect of the present invention, a magnetic incremental motion detection system with offset compensation comprises a target wheel having a plurality of uniformly circumferentially spaced indications adjoined thereto, a magnetic sensor positioned adjacent the target wheel with an air gap therebetween, a linear amplifier having first and second inputs the first of which is connected to the magnetic sensor, and a low-pass filter connected between the magnetic sensor and the second input of the amplifier.
According to another aspect of the present invention, a magnetic incremental motion detection system with offset compensation comprises a target wheel having more than two uniformly circumferentially spaced indications adjoined thereto, a magnetoresistive sensor positioned adjacent the target wheel with an air gap therebetween, an amplifier having an input connected to the magnetic sensor, a signal comparison stage connected to the amplifier for producing a digital signal in response to the analog output signal of the amplifier, and means for compensating for offset in the output signal of the magnetoresistive sensor.
According to a further aspect of the present invention, a magnetic incremental motion detection system with offset compensation comprises a target wheel having a plurality of uniformly circumferentially spaced indications adjoined thereto, a magnetic sensor positioned adjacent to the target wheel with an air gap therebetween, an amplifier having an input connected to the magnetic sensor, a signal comparison stage connected to the amplifier for producing a digital signal in response to the analog output signal of the amplifier, and means connected between the magnetic sensor and the amplifier for compensating for offset in the output of the magnetic sensor.